Various seam welding methods of metallic fuel tanks for automobiles have conventionally been proposed. For example, the following Patent Document 1 discloses a method of making a fuel tank supported by a multi-axial robot, and sandwiching and welding a flange portion for joining of a fuel tank between upper and lower electrode rings of a seam welder installed in place. In this method, the periphery of the flange portion for joining is continuously seam-welded while the posture of the fuel tank is rotated. Thereby, complicated jigs can be made unnecessary, fuel tanks of different shapes can also be coped with easily, and wear of the lower electrode ring can be automatically corrected in a follow-up manner.
Further, the following Patent Document 2 discloses a seam welding line copying automatic apparatus in which an XY table which is a supporting device of a fuel tank which permits horizontal movement, but constrains a motion in a horizontal turning direction, and a copying control shaft having a main shaft in a direction orthogonal to a movement permissible direction of this XY table are superposed up and down. In this seam welding line automatic copying apparatus, expensive copying jigs required for seam welding, complicated programs for robots which support workpieces, etc. are made unnecessary.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H07-241684    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H09-314345